1. Field of the Invention
The present invention relates to a zoom lens suitable for a camera such as a digital still camera or a digital video camera, and more particularly to a portable zoom lens with reduced variation in exit pupil positions during zooming, an increased angle of view, and a reduced total length of lenses.
2. Description of the Related Art
Digital still cameras and digital video cameras using a solid-state image pickup device have recently become multifunctional, and optical systems used therein have needed both high optical performance and reduced size.
Generally, a zoom lens used for a camera with a CCD such as a video camera is of a type including three to five lens units, a front lens unit among them consisting of lenses having positive, negative and positive refractive power, and a lens unit closest to an object being fixed during zooming.
For example, Japanese Patent Laid-Open No. 63-81313 proposes a zoom lens including four lens units having positive, negative, positive, and positive refractive power, and having an approximately 3× zoom ratio. Also, for example, Japanese Patent Laid-Open No. 3-296706 proposes a zoom lens with a higher zoom ratio that includes four lens units having positive, negative, positive, and positive refractive power, and having an appropriately 10× zoom ratio.
On the other hand, as a zoom lens for an electronic still camera for taking still images with a CCD, an optical system is required that has an extremely short total length of lenses, has a wide angle of view suitable for properties of the still images, and further has higher optical performance than a zoom lens used for a video camera for moving images.
As a lens including a wide angle area and achieving bright and high performance even with a relatively low zoom ratio of approximately 2.5 to 3×, Japanese Patent Publication No. 6-66008 or the like proposes a zoom lens including two lens units having negative and positive refractive power, zooming is performed by changing an air space of each lens unit.
Japanese Patent Laid-Open No. 7-82256 proposes a zoom lens including three lens units having negative, positive, and positive refractive power, a space between a second unit and a third unit increasing during zooming from a wide angle end to a telephoto end.
U.S. Pat. No. 5,434,710 discloses a zoom lens including three lens units having negative, positive, and positive refractive power, a space between a second unit and a third unit decreasing during zooming from a wide angle end to a telephoto end.
Japanese Patent Laid-Open No. 60-31110 proposes a zoom lens including four lens units having negative, positive, positive, and positive refractive power, a space between a second unit and a third unit decreasing, and a fourth unit being fixed during zooming from a wide angle end to a telephoto end.
Japanese Patent Laid-Open No. 10-104520 proposes discloses a zoom lens of a four-unit configuration including lens units having negative, positive, positive, and positive refractive power.
Japanese Patent Laid-Open No. 11-84242 proposes a zoom lens of a four unit type including lens units having negative, positive, positive, and positive refractive power, the zoom lens having a compact body, an approximately 3× zoom ratio, and relatively small exit pupil variation during zooming.
Multi-pixel solid-state image pickup devices have been recently increasing, and image taking lenses have needed higher optical performance than that of conventional lenses.
Especially, a zoom lens for a solid-state image pickup device is required that has telecentricity (an exit pupil position being remote from an image plane (if possible, infinity)) necessary for reducing shading, and has a reduced total length of lenses, a compact body, and a high zoom ratio.
A zoom lens with large exit pupil variation of a lens system during zooming can be used as a zoom lens for a silver camera, but when used for an electronic still camera with a CCD, power distribution is limited from a design viewpoint to increase a size of the total lens system.
In a zoom lens of a “negative lead type” in which a lens unit having negative refractive power is placed closest to an object, it is generally necessary to appropriately set refractive power distribution of each lens unit, a lens configuration of each lens unit, and a refractive index or an Abbe number of a material of each lens, in order to reduce the number of lenses of a total lens system, simplify a lens configuration, and achieve an increased angle of view and good optical performance across a zoom range. If the refractive power distribution of each lens unit, the lens configuration, and the material of the lens are inappropriately selected, aberration variation, particularly chromatic aberration variation in accordance with zooming increases to cause difficulty in achieving high optical performance across the zoom range.
For example, in the zoom lens of the conventional negative lead type, a positive lens in a first lens unit has been generally made of a glass material having an Abbe number of 24 or more. Thus, if the first lens unit is to consist of two lenses: a negative lens and a positive lens, the negative lens requires an aspheric lens with a large deflection ratio between center thickness and edge thickness to cause difficulty in manufacturing.